DESCRIPTION: One-carbon (Cl) metabolism consists of the generation of carbon units for use in cellular processes including DNA synthesis, regeneration of methionine (Met) from homocysteine (Hcy), and methylation of many biological compounds. Conditions that impair one-carbon metabolism (e.g. folate deficiency) are associated with elevation in plasma Hcy and increased risk of vascular disease, certain cancers, and neural tube defects. A common mutation of methylene-tetrahydrofolate reductase (MTHFR), known as the "thermolabile" or C677T mutant, has been associated with elevations in plasma I-Icy (especially in low folate status), lower plasma folate, altered distribution of erythrocyte folate, potentially increased risk of vascular disease, and decreased risk of colon cancer. The in vivo metabolic effects of the C677T mutation have not been determined directly. Our overall hypothesis is that the rate of acquisition and generation of methyl groups from serine (primary source of C1 units) is reduced in individuals homozygous for the C677T mutation, and that the genotypic effect is greatest when folate nutriture is inadequate. We also hypothesize that the rate of folate-dependent synthesis of nucleotides (purines and thymidylate) will be reduced in folate deficiency but may be enhanced by the C677T mutation. The proposed studies will determine nutritional and genetic dependence of the flow of Cl units from serine (Ser) to Met and from Ser to nucleotides. This protocol also will allow measurement of the transsulfuration pathway of Hcy catabolism important in disposal of excess Hcy. Specific aims. To determine: (a) The kinetics by which Ser serves as a donor of Cl units for methyl group synthesis and nucleotide synthesis and the possible degree of impairment caused by the C677T mutation and/or low folate status. (b) The influence of the C677T mutation and folate status on cellular Cl status as reflected by the distribution of folate species in erythrocytes. (C) The influence of the C677T mutation and folate status on homocysteine catabolism. (d) The relative contributions of cytosolic and mitochondrial metabolism in the generation of Cl units for synthesis of methyl groups and nucleotides. (e) The significance of mitochondrial glycine cleavage in generation of Cl units. Protocol: In the main protocol, healthy adequately nourished human subjects (20-30 yr) will be classified by MTHFR genotype, (homozygous control and homozygous mutant). Subjects will be given infusions with 13C-serine as primary precursor initially and following 8-wk dietary depletion of 120 ugld folate to evaluate the effect of nutritional and genotypic effects on Cl kinetics. Two variations of this study will be conducted to determine the relative roles of mitochondrial and cytosolic routes of Cl generation from serine and the role of the mitochondrial glycine cleavage pathway. In total, these studies will yield new functional data regarding the effects of folate deficiency, and the influence of common polymorphism of MTHFR.